1. Field of the Invention
This invention relates to a composite substrate material comprising at least two substrates which are bonded to each other. This invention also relates to a process for producing the composite substrate material.
2. Description of the Related Art
Composite materials have heretofore been formed by bonding optical crystals, such as laser crystals, optical wavelength converting crystals, and quartz glass mirrors, to each other or by bonding optical substrates to each other. As techniques for the bonding, techniques, wherein the optical crystals or the optical substrates are adhered to each other by optical adhesive agents, or techniques, wherein the optical crystals or the optical substrates are fusion bonded under heat, have heretofore been used widely.
However, the techniques, wherein the optical crystals or the optical substrates are adhered to each other by optical adhesive agents, have the problems in that optical scattering and reflection loss are caused to occur and long-term reliability of the adhered areas is low. Particularly, as for optical members to be located within laser resonators, the problems described above arise markedly.
The techniques, wherein the optical crystals or the optical substrates are fusion bonded under heat, have the problems in that the techniques are applicable only to limited combinations of materials and can be utilized only in limited applications.
Therefore, recently, techniques for bonding different kinds of materials to each other, which techniques are referred to as wafer bonding techniques, have attracted particular attention. With the wafer bonding techniques, as described in, for example, Japanese Unexamined Patent Publication No. 6(1994)-90061, wafers of single crystals or polycrystals are subjected to mirror finish, the mirror surfaces thus obtained are washed to remove dust and organic substances and are set in a hydrophilic state, the mirror surfaces are then brought into contact with each other in a clean atmosphere, and the wafers are heated in this state.
With the wafer bonding techniques described above, a composite substrate material. having a high bond strength can be formed. However, the wafer bonding techniques have the problems in that wet processing must be performed for cleaning the substrates and therefore production steps cannot be kept simple.
The primary object of the present invention is to provide a process for producing a composite substrate material, wherein wet processing need not be performed, and a composite substrate material having a high bond strength, high long-term reliability of a bonded area, and good environmental temperature resistance characteristics is capable of being produced.
Another object of the present invention is to provide a composite substrate material having a high bond strength, high long-term reliability of a bonded area, and good environmental temperature resistance characteristics of the bonded area.
The present invention provides a process for producing a composite substrate material, comprising the steps of:
i) overlaying a functional film, which has photo-catalytic effects, on a surface of each of at least two substrate materials,
ii) exposing the functional film, which has been overlaid on the surface of each of the at least two substrate materials, to light having a wavelength falling within an absorption wavelength range of the functional film, and
iii) bonding the at least two substrate materials to each other with the functional films, which have been exposed to the light, intervening between the at least two substrate materials.
In the process for producing a composite substrate material in accordance with the present invention, as the functional film, a film containing TiO2 should preferably be employed. In such cases, the light irradiated to the functional film should preferably be ultraviolet light. Also, in cases where a film other than the functional film is also overlaid on the surface of each of the at least two substrate materials, the functional film should preferably be formed as a top layer.
The present invention also provides a composite substrate material produced with the process in accordance with the present invention.
Specifically, the present invention also provides a composite substrate material, comprising at least two substrate materials bonded to each other with functional films, which have photo-catalytic effects, intervening between the at least two substrate materials. In the composite substrate material in accordance with the present invention, each of the functional films should preferably be a film containing TiO2.
Also, in the composite substrate material in accordance with the present invention, each of the functional films should preferably also act as an anti-reflection film. Further, each of the functional films may be formed thin such that each of the functional films is optically negligible.
With the process for producing a composite substrate material in accordance with the present invention, when each of the functional films having the photo-catalytic effects, such as metal oxide films, typically TiO2, is exposed to the light having a wavelength falling within the absorption wavelength range of the functional film, the photo-catalytic effects of the functional film occur. As a result, organic substances, and the like, which cling to the surface of each of the substrate materials, are decomposed approximately perfectly, and a super-hydrophilic state occurs on the surface of each of the substrate materials. Thereafter, the substrate materials are combined with each other such that the surfaces of the substrate materials, which surfaces are provided with the functional films, stand facing each other. Also, a load and heat are applied to the combined substrate materials. In this manner, the substrate materials are bonded to each other with a high bond strength. The bonded area has high long-term reliability and good environmental temperature resistance characteristics.
Further, with the process for producing a composite substrate material in accordance with the present invention, wherein wet processing need not be performed, the effects described above can be obtained easily.
Particularly, with the process for producing a composite substrate material in accordance with the present invention, wherein each of the functional films also acts as the anti-reflection film, a composite substrate material, which is substantially free from reflection loss at the bonded area or exhibits little reflection loss at the bonded area, can be obtained. Also, with the process for producing a composite substrate material in accordance with the present invention, wherein each of the functional films is formed thin such that each of the functional films is optically negligible, a composite substrate material, which is substantially free from scattering loss at the bonded area or exhibits little scattering loss at the bonded area, can be obtained.
In this manner, for example, a reflectivity at the bonded area of a plurality of optical substrates (or crystals) can be limited easily and reliably to a value of approximately at most 0.2%. Also, it becomes possible to directly bond laser crystals, optical wavelength converting crystals, or other crystals in solid lasers, and the like, to each other. Therefore, with the process for producing a composite substrate material in accordance with the present invention, a subminiature solid laser having reliable performance can be furnished.